After drilling an oil or gas well, a metal casing is conventionally cemented to isolate the producing zones. The wall of this casing may be perforated at zones where the entry of oil or gas from the surrounding formation is desired. This activity may be referred to as jet perforation. Charges may be detonated by making electric current pass through an electrical initiator, also known as a detonator, igniter or blasting cap.
A number of charges may be assembled inside a steel hollow carrier. Sometimes a zone uses a number of perforations along a predetermined length, defining a perforating density which uses a selected number of charges inside a hollow carrier of a known length. For lower cost and rig time, it is desirable to perforate several different production zones using one wireline trip into the hole, rather than make one trip for each perforating operation.
Dual or tandem selective guns have long been used which perforate two different zones by using diodes. By applying a positive voltage, the bottom section may be detonated, and then the top section on next zone detonated applying a negative voltage.
Conventional prior art methods involve the use of alternating polarity, i.e., one section is fired by a positive voltage, the next section will require a negative voltage, and so on. The condition to keep the sequence going requires that the next upper section gets a complete explosive detonation, meaning that the electric initiator and the intended charge or group of charges in the lower section must successfully go off. A mechanical switch may be used to electrically connect one of the leads of the electrical initiator, usually the ground cable, for the next section to be fired, thereby requiring mechanical energy coming from the explosion of the charges.
Rather than use the mechanical force of a detonating wave to activate the switch for firing the next gun, some systems utilize a motorized switch with mechanical contacts. Motorized switches have high maintenance costs and complicated operational procedures, and frequently have a high failure rate due to mechanical nature of operation. Selector switches have also been devised that utilize digital codes to select the desired gun to be fired. Selector switches that utilize digital codes require special fire control panels.
Systems which rely upon mechanical switching techniques have inherent disadvantages. The shock wave from a large gun may be sufficient to disable any mechanical switching system. Accordingly, mechanical switching systems have generally been limited to applications with guns with relatively small charges. Other systems involve multiple lines for detonation of multiple guns, but these multiple conductor lines add appreciable costs.
U.S. Pat. Nos. 5,531,164 and 5,700,969 disclose systems which control a selected firing detonation. A switch may be activated by a shock wave acting on a dart which provides an electrical path to ground. The ground path then results in altering the state of the switch, i.e., a path to ground or a path isolated from ground. These patents rely upon the repercussion shock of one charge to respectively “cock” the system for firing the next gun. Since it is not uncommon for one of the guns not to fire (which may occur in from 5 to 10% of the cases), misfiring of one gun requires that the entire string be pulled. Also, a gun may fire so strongly that the cable, which is typically pressed into engagement with ground, may be cut so that the signal is lost, and again the string must be returned to the surface.
Other systems for activating a perforating guns are disclosed in U.S. Pat. Nos. 4,208,966, 4,234,767, 4,454,814, 4,457,383, 4,496,010, 4,991,684, 4,527,636, 4,778,009, 5,521,164, and 5,700,969. PCT/US90/01842 discloses a geophysical exploration system using large explosive charges.
The disadvantages of the prior art are overcome by the present invention, and an improved perforated system and method are hereinafter disclosed. The invention may also be used to selectively fire a downhole sampling tool.